Typically, particle detectors employ laser devices producing a beam of coherent light that is directed to impinge upon a sample of particles. Particle detection is achieved either by sensing light scattered by a particle, e.g. U.S. Pat. No. 5,085,500 to Blesener et al., or by detecting extinction of light, e.g. U.S. Pat. No. 5,121,988 to Blesener et al. U.S. Pat. No. 4,811,349 to Payne et al. discloses a chromium colquiriite such as Cr:LiSrAlF.sub.6 as a useful laser crystal. U.S. Pat. No. 5,105,434 to Krupke et al. discloses a chromium colquiriite solid-state laser such as Cr:LiSrAlF.sub.6 pumped with at least one AlGaAs semiconductor laser, and U.S. Pat. No. 5,249,189 to Scheps discloses a chromium colquiriite solid-state laser such as Cr:LiSrAlF.sub.6 pumped with at least one visible AlGaAs semiconductor laser. U.S. Pat. No. 5,317,447 to Baird and DeFreez discloses, in pertinent part, a solid state laser having a Cr:LiSrAlF.sub.6 crystal pumped from one end by an array of semiconductor laser diodes and suggests use of broad area laser diodes.
Particle detectors have been used for a variety of purposes to detect the presence and/or size of particles in various fluids, including air and other gases, as well as liquids, such as water, hydraulic oils and lubricants. They have proved particularly useful to control contamination in many industrial environments. For example, particulate contamination can cause hydraulic equipment and the like to fail due to excessive accumulation of particles in the hydraulic fluid. Even though filters are used in such equipment to continuously remove particles, the filters may become clogged and may rupture due to excess pressure build-up across the filter membrane. Also, microelectronic fabrication requires a "clean room" in which particulate contaminants, e.g., dust, are filtered from an atmosphere of a room. The filters used in "clean rooms" are also subject to clogging and compromise, resulting in particulate matter entering a "clean room" atmosphere in great quantities. Failure to provide a "clean room" results in particulate contamination of the devices during fabrication, which reduces yield. Particle detectors are thus used in such environments to detect particles in specified size classes and report the cleanliness level of the fluid according to categories specified by industry standards.
A significant amount of research has been performed using open cavity gas lasers in particle detection systems and is discussed by R. G. Knollenberg and B. Schuster in "Detection and Sizing of Small Particles in an open Cavity Gas Laser", Applied Optics, Vol. 11, No. 7, pp. 1515-1520 (November 1972). Sub-micron particle sizing devices utilizing light scattering in an open cavity laser device is described by R. G. Knollenberg and R. E. Leur in "Open Cavity Laser `Active` Scattering Particle Spectrometry from 0.05 to 5 Microns", Fine Particles, Aerosol, Generation Measurement, Sampling and Analysis, Academic Press, pp. 669-696 (May 1975).
U.S. Pat. No. 4,594,715 to Knollenberg discloses an external cavity gas laser for use as a particle detector that includes first, second and third spaced mirrors. The first and second mirrors define an active resonant cavity of a gas laser, and the second and third mirrors define a second cavity. The second cavity ranges between being passive and being closely coupled as part of the active cavity, depending on the phase of the light returned from the third mirror to the second mirror. In the limiting case where the second cavity is not resonant, a large field does not build up in the passive cavity, because that cavity is off resonance for the wavelength of the active cavity. In the latter case, the second mirror, ignoring scattering and absorption losses in its coating and substrate, becomes transparent to light recirculating in the external cavity formed by the first and third mirrors, thus destabilizing the original cavity modes of the resonator formed by the first and second mirrors. In this case, even a small amount of absorption or scattering in the coating or substrate of the second mirror will result in the resonator formed by the first and third mirrors to have low net gain. This is because the inherent low gain of many types of gas lasers renders them particularly sensitive to intracavity loss. To address these issues, Knollenberg describes a method to modulate the external cavity along the laser axis, thereby creating a broad, Doppler induced, incoherent spectrum. This reduces the Q value of that cavity, because the nominal Q, as calculated from standard Fabry-Perot formulas, depends, for buildup of optical power, on having a resonance wavelength as opposed to a broad spectrum.
A problem with certain gas lasers is that they sometimes have a highly charged window, most of the time a Brewster window. A high voltage plasma inside of the tube creates a charge build-up on the outside of the window because the window is a very good insulator. This charge attracts oppositely charged molecules that get through purge filters. After a period of time (from 2 days to 2 weeks) depending on concentration, the build-up is a high enough loss that it causes loss in lasing power and inevitable catastrophic failure.
It is an object of the present invention to provide a resonant cavity type particle detector having improved efficiency with an optimized resonant cavity and which is not subject to the catastrophic failure mode of a gas laser.